1. Field of the Invention
The present invention relates to a method of treating substances by applying discharge plasma to a fluid containing the substances to be treated. The present invention also relates to an apparatus for carrying out such a substance treating method. Particularly, the present invention relates to a technique for decomposing, by discharge plasma, harmful or poisonous substances such as dioxins contained in waste gases emitted from burning systems into harmless substances or easily collectable substances.
2. Description of the Related Art
Various kinds of harmful substances are contained in waste gases discharged from city type incinerators and large scale industrial waste treating plants. Recently, not only NOx and SOx, but also dioxins have been classified as harmful substances. It is important to emit the waste gas into the atmosphere only after reducing the concentrations of these harmful substances below the allowable safety level.
Heretofore, many treating methods have been developed for reducing concentrations of harmful substances in waste gases. However, in the known methods, large scale treating equipment is required, a treating efficiency is relatively low, running costs are rather high, and maintenance is complicated. Therefore, the known treating methods could not be effectively used. For instance, in many burning systems, electric precipitation devices are used. However, it has been made clear that the electric precipitation itself might be a source generating harmful dioxins. Therefore, a bag filter has been utilized instead of the electric precipitation device. However, the bag filter has a short life time and its maintenance is rather complicated.
In order to remove or mitigate the above-mentioned demerits, there has been proposed a substance treating method in which harmful substances are reacted with electrons generated by corona discharge or dielectric barrier discharge and are transformed or converted into harmless substances or substances that could be collected easily. FIG. 1 shows a known waste gas treating apparatus. An electrically conductive pipe 1 serving as a coaxial tubular reaction vessel is provided, a wire electrode 2 is arranged along a longitudinal axis of the pipe 1 and a pulse supply source 3 is connected across the pipe and the wire electrode to generate corona discharge within the pipe. Then, a waste gas is flowed through the pipe 1 and dioxins, NOx and SOx are reacted with radicals and accelerated electrons generated by the corona discharge and are decomposed into harmless substances.
FIG. 2 shows another known waste gas treating apparatus using the above-mentioned pulse discharge plasma. In this apparatus, an inner pipe 4 made of a dielectric material is arranged on an inner wall of an electrically conductive outer pipe 1 and a wire electrode 2 is arranged within the inner pipe 4 along its longitudinal axis. In this apparatus, an AC voltage supply source 5 is connected across the electrically conductive outer pipe 1 and the wire electrode 2 to generate the barrier discharge.
FIG. 3 is a perspective view depicting another known waste gas treating apparatus disclosed in Japanese Institute of Electrical Engineers Theses, Hiroyuki YASUI, xe2x80x9cWaste Gas Treating Technique with Pulse Corona Discharge,xe2x80x9d Vol. 119, No. 5, 1997. A plurality of plate electrodes 6 are arranged in parallel with each other, wire electrodes 7 are arranged between adjacent plate electrodes, and a pulse supply source 3 is connected across the plate electrodes 6 and the wire electrodes 7. A waste gas containing substances to be treated is flowed through spaces between successive plate electrodes 6.
In the known substance treating method using the discharge plasma, the wire electrode is arranged along the longitudinal axis of a gas flow passage having a relatively large cross sectional area, and therefore, the discharge plasma could not be generated uniformly along the space through which the waste gas flows. For instance, in the known apparatus shown in FIG. 1, the discharge plasma is locally produced around the wire electrode 2 as depicted in FIGS. 4 and 5, and the plasma is scarcely generated near the periphery of the gas flow passage. When the discharge plasma is localized, a possibility of the reaction of given substances contained in the waste gas with electrons generated by the plasma might be decreased, and the treating efficiency becomes low. This problem also occurs in the known apparatus shown in FIG. 3 using the plate electrodes 6 and wire electrodes 7.
In the above explained known substance treating apparatuses, the pulse supply source or AC voltage supply source is connected across the two kinds of electrodes. In order to decompose dioxins contained in a waste gas, dioxins have to be reacted with electrons having substantially high energy. However, the known waste gas treating apparatuses could not efficiently generate electrons having high energy. That is to say, although an ordinary AC voltage supply source is connected across the electrodes, it is impossible to generate efficiently electrons having desired high energy.
When the AC voltage supply source is utilized, electrons having energy of about 1 eV are predominantly generated as shown by a curve A (HF plasma) in FIG. 6, but a density of electrons having energy higher than 5 eV becomes very low. In order to decompose dioxins efficiently, electrons having energy of about 3-10 eV are effective, but a density of such high energy electrons is low. Therefore, the known apparatus using the high frequency voltage supply source could not decompose dioxins efficiently.
Moreover, in case of using the pulse supply source, in order to decompose dioxins efficiently, it is necessary to generate electrons having energy of about 3-10 eV. To this end, a voltage pulse applied across the discharge electrodes must have a sharp or steep raising edge as well as a short pulse duration. To this end, one may consider to utilize a pulse supply source including a thyratron as an active element. The pulse supply source using the thyratron has sharp raising edge, short pulse duration and large discharge current as shown in FIG. 7. However, the thyratron has several drawbacks such as large size, low power efficiency, high cost, short life time, large secular variation and cumbersome maintenance. Particularly, in the waste treating system, the power consumption is liable to be large due to the cathode heater and the cost for exchanging the thyratron having a short life time becomes high.
In order to overcome the above mentioned problems, it is desired to utilize a pulse supply source having a semiconductor element as a switching element, because the semiconductor element has a high power efficiency and a very long life time. As the semiconductor switching element, use may be made of a GTO (Gate Turn-off Thyristor) and an IGBT (Insulated Gate Bipolar Transistor). However, a GTO has a very slow raising edge and a long duration as illustrated in FIG. 8. Furthermore, electrons having a desired energy level could not be generated efficiently unless a plurality of large scale circuits including magnetic compression circuits are connected in cascade. Although an IGBT can generates a pulse having a steeper raising edge and a shorter duration than a GTO, as shown in FIG. 9, its raising edge is not sufficiently sharp to generate electrons having high energy of about 3-10 eV with a high density, such electrons being necessary for decomposing dioxins in an efficient manner.
There has been also proposed to treat harmful substances contained in waste gases using materials having catalysis. However, the treating efficiency is not sufficiently high, and waste gases emitted from city type incinerators and industrial waste treating plants could not be purged effectively, because such waste gases contain harmful substances with very high densities.
Recently it has been proposed to treat harmful substances using materials such as TiO2 having photocatalysis. However, this type photocatalysis needs to be excited with ultraviolet radiation, and therefore it is limited to outdoor applications such as load fences, load surfaces and outer walls of buildings. Therefore, photocatalysis could be not be utilized for purging waste gases emitted from city type incinerators and industrial waste treating plants.
The object of the present invention is to provide a novel and useful method of treating substances, in which the substances can be treated efficiently by a combination of catalysis and discharge plasma generated uniformly along a long passage through which a fluid containing the substances to be treated flows.
It is another object of the invention to provide a method of treating substances, in which the substances can be treated efficiently with a pulse discharge plasma which can generate electrons having a desired high energy level with a high density.
It is another object of the invention to provide a method of treating substances, in which the above mentioned pulse discharge plasma generating electrons having a high energy level can be effectively produced by a semiconductor switching element.
It is still another object of the invention to provide a method of treating substances, in which a treating efficiency can be improved by exciting a photocatalysis material with ultraviolet radiation emitted from the discharge plasma.
According to the present invention, a method of treating substances comprises the steps of:
generating discharge plasma within an electrically insulating honeycomb structural body having a plurality of through holes by applying a discharge voltage across electrodes, at least a part of the electrodes being made of a metal having catalysis;
flowing a fluid containing substances to be treated through the plurality of through holes formed in the honeycomb structural body; and
treating the substances contained in the fluid by a reaction with the discharge plasma and by catalysis of at least a part of the electrodes.
According to further aspect of the invention, a method of treating substances comprises the steps of:
generating discharge plasma within an electrically insulating honeycomb structural body having a plurality of through holes by applying a discharge voltage across electrodes, at least a part of the honeycomb structural body being made of a material having photocatalysis;
flowing a fluid containing substances to be treated through the plurality of through holes formed in the honeycomb structural body; and
treating the substances contained in the fluid by a reaction with the discharge plasma and by decomposition and/or oxidation by active oxygen generated by the photocatalysis material excited with radiation emitted from the discharge plasma.
According to further aspect of the invention, a method of treating substances comprises the steps of:
generating discharge plasma within an electrically insulating honeycomb structural body having a plurality of through holes by applying a discharge voltage across electrodes, at least a part of the honeycomb structural body being made of a photocatalysis material and at least a part of the electrodes being made of a metal having catalysis;
flowing a fluid containing substances to be treated through the plurality of through holes formed in the honeycomb structural body; and
treating the substances contained in the fluid by a reaction with the discharge plasma, by catalysis of at least a part of the electrodes and by active oxygen generated by the photocatalysis material excited with radiation emitted from the discharge plasma.
The present invention also relates to an apparatus for treating substances, and has for its object to provide a substance treating apparatus which can treat particular substances efficiently, while the apparatus can be small in size, can save power, can have a long life time, and is less expensive in cost.
According to the present invention, an apparatus for treating substances comprises:
an electrically insulating honeycomb structural body having a plurality of parallel through holes through which a fluid containing substances to be treated flows;
an electrode system for generating discharge plasma within the honeycomb structural body such that the substances contained in the fluid flowing through the through holes is reacted with the discharge plasma, at least a part of the electrode system being made of a material having catalysis; and
a power supply source connected to the electrode system for applying a discharge voltage to the electrode system such that the discharge plasma is generated within the through holes of the honeycomb structural body.
According to further aspect of the invention, an apparatus for treating substances comprises:
a honeycomb structural body having a plurality of parallel through holes through which a fluid containing substances to be treated flows, at least a part of the honeycomb structural body being made of ceramics including a material having photocatalysis;
an electrode system for generating discharge plasma within the honeycomb structural body such that the substances contained in the fluid flowing through the through holes are reacted with the discharge plasma and the material having photocatalysis is excited with radiation emitted from the discharge plasma; and
a voltage supply source connected to the electrode system for applying a discharge voltage to the electrode system such that the discharge plasma is generated within the through holes of the honeycomb structural body.
According to further aspect of the invention, an apparatus for treating substances comprises:
an electrically insulating honeycomb structural body having a plurality of parallel through holes through which a fluid containing substances to be treated flows, at least a part of the honeycomb structural body being made of a material having photocatalysis;
an electrode system for generating discharge plasma within the honeycomb structural body such that the substances contained in the fluid flowing through the through holes is reacted with the discharge plasma and the material having photocatalysis is excited with radiation emitted from the discharge plasma, at least a part of the electrode system being made of a material having catalysis; and
a power supply source connected to the electrode system for applying a discharge voltage to the electrode system such that the discharge plasma is generated within the through holes of the honeycomb structural body.
In the method and apparatus for treating a substance according to the present invention, gas or liquid, i.e. fluid containing substances to be treated flows though the through holes formed in the honeycomb structural body and the discharge plasma is generated within the through holes. Since the discharge space is defined by the through holes, the discharge plasma is generated uniformly over the whole cross-sectional area of the fluid passages, and thus the substances can be efficiently reacted with radicals and electrons generated by the discharge plasma. In this manner, the substance treating efficiency can be increased.
Furthermore, when at least a part of the electrode system for generating the discharge plasma is made of a catalysis metal such as platinum, palladium and nickel series metals, the substances to be treated can be decomposed by the reaction with electrons having a lower energy level under the catalysis. In this manner, the treating efficiency can be further improved materially.
Moreover, when at least a part of the honeycomb structural body is made of a photocatalysis material, the photocatalysis material can be effectively excited with ultraviolet radiation emitted from the discharge plasma which is generated uniformly along the through holes of the honeycomb structural body. Then, active oxygen is generated, and the substances to be treated are decomposed and/or oxidized by the thus generated active oxygen. According to the present invention, the active oxygen is generated not only near the inner wall of the through hole, but also over the whole cross-sectional area of the through hole, and therefore the treating efficiency is very high. Further, the substances to be treated are first transformed into intermediate substances by radicals and electrons in the discharge plasma, and then the thus transformed intermediate substances are further treated by the active oxygen. Alternatively, the substances to be treated are first reacted with the active oxygen to produce intermediate substances, and then the thus transformed intermediate substances are reacted with radicals and electrons in the discharge plasma. In this manner, the substances contained in the fluid can be treated very efficiently.
The method and apparatus for treating substances according to the present invention may be used for various applications. Particularly, it is preferable to apply the present invention to city type waste incinerators and large scale industrial waste treating plants, and dioxins, NOx and SOx contained in waste gases discharged from these burning systems can be decomposed into harmless substances by the reaction with the discharge plasma generated within the honeycomb structural body. In such applications, it is particularly preferable to generate the discharge plasma within the honeycomb structural body as pulse corona discharge plasma. In such a pulse corona discharge plasma, electrons having sufficiently high energy level for effectively decomposing dioxins can be generated with an extraordinary high density. In this case, in order to decompose harmful substances such as dioxins in an effective manner, it is preferable to generate the pulse corona discharge plasma which can produce electrons having high energy level of 3-10 eV. To this end, it is preferable that a raising edge of a pulse discharge current is larger than 5xc3x971010 A/second, particularly larger than 1xc3x971011 A/second and an amplitude of the pulse discharge current is several thousands amperes.
According to the present invention, in order to generate the pulse corona discharge plasma, it is preferable to construct the pulse supply source by a static induction thyristor as a switching element. The static induction thyristor is a semiconductor switching element, and therefore the size is small, power consumption is low, the life time is semi-permanent, maintenance is easy, and costs can be reduced.
According to a first principal structure of the present invention, the discharge plasma is generated within the honeycomb structural body in a direction parallel to a longitudinal direction of the through holes, and according to a second principal structure of the present invention, the discharge plasma is generated within the honeycomb structural body in a direction perpendicular to a longitudinal direction of the through holes.
In a preferred embodiment of the substance treating apparatus according to the first principal structure of the present invention, the electrode system comprises first and second electrodes provided on respective end surfaces of the honeycomb structural body and the first and second electrodes are connected to the power supply source such that a discharge voltage is applied in a direction parallel to a longitudinal direction of the through holes. In such a structure, the first and second electrodes may be formed by first and second mesh electrodes provided on the end surfaces of the honeycomb structural body, or may be formed by metal films applied on the end surfaces of the honeycomb structural body. In the later case, it is preferable that the metal films are extended onto inner walls of the through holes.
In a preferred embodiment of the substance treating apparatus according to the second principal structure of the present invention, the electrode system comprises a cylindrical electrode arrange on the honeycomb structural body and a plurality of wire electrodes passing through holes, the cylindrical electrode being connected to a first output terminal of the power supply source and the plurality of wire electrodes being connected to a second output terminal of the power supply source.
In another preferable embodiment of the substance treating apparatus according to the second principal structure of the present invention, the electrode system comprises first and second groups of a plurality of wire electrodes passing through the through holes, the first and second groups of a plurality of wire electrodes being connected to first and second output terminals, respectively of the power supply source.
In another preferred embodiment of the substance treating apparatus according to the second principal structure of the present invention, the electrode systems comprises a first group of a plurality of strip electrodes each being applied on inner walls of the through holes and a second group of a plurality of strip electrodes each being applied on the inner walls of the through holes to be opposed to the first group strip electrodes, the first and second groups of a plurality of wire electrodes being connected to first and second output terminals, respectively of the power supply source.
In case of using the honeycomb structure, it is preferable to form protrusions and depressions in inner walls of the through holes of the honeycomb structural body. Then, the fluid containing the substances to be treated becomes a turbulent flow and is effectively stirred.
According to the present invention, a plurality of honeycomb structural bodies are arranged in parallel with each other or in series with each other. In the former case, electrodes arranged on one end surfaces of the honeycomb structural bodies are commonly connected to one output terminal of the power supply source and all electrodes provided on the other end surfaces of the honeycomb structural bodies are commonly connected to the other output terminal of the power supply source. In the later case, electrodes arranged on end surfaces of respective honeycomb structural bodies may be connected to a same power supply source or different power supply sources having different output voltages.
According to further aspect of the present invention, an apparatus for treating substances comprises:
a sleeve electrode;
a first insulating sleeve made of ceramics containing a material having photocatalysis and arranged in an inner wall of the sleeve electrode, the first insulating sleeve constituting a passage for a fluid containing substances to be treated;
a wire electrode arrange along a central axis of the first insulating sleeve;
a second insulating sleeve made of ceramics containing a material having photocatalysis and arranged around the wire electrode; and
a discharge voltage source connected to the sleeve electrode and wire electrode to generate discharge plasma between the sleeve electrode and the wire electrode;
wherein the substances contained in the fluid are treated by reaction with the discharge plasma and with active oxygen generated by exciting the material having photocatalysis with radiation emitted from the discharge plasma.
Also in this substance treating apparatus, it is preferable that the discharge voltage source is formed by a pulse supply source to generate pulse corona discharge between the sleeve electrode and the wire electrode. Furthermore, the material having photocatalysis may be TiO2.